Sorry if this question has been discussed before, but how long does a barrel need to be to be efficient for BP cartridge shooting? I know it depends on bore size and how much powder is to be consumed, but bear with me. Both sporting and military BP cartridge barrels were commonly 30 inches to as long as 34 or more inches, but I suspect extremely long barrel lengths weren't needed for powder burn efficiency as much as to obtain a long sight radius, dampen recoil, and serve as a pike when a bayonet was mounted.
The Trapdoor M1873 Springfield was offered in 3 nominal barrel lengths, a 22" carbine, a 26" "Officer's Model", and the standard infantry version with about a 32 inch barrel. How efficient were the two shorter versions with the standard full power service cartridge with either a 405 or 500 grain bullet over 70 grains of 1f or 2F black powder?
I'm planning a .50-70 project and haven't determined how long I want the barrel to be. I have rifles in my gun safe with barrels as long as 37" and find them a bit unwieldy. I'm considering building a carbine or short rifle with a barrel between 22-26 inches, using "real" FFg black powder and a Lyman 515141 (~425 grains) as a standard load. Would the shorter barrel lengths allow for efficient burning of a full case of BP? How about with Pyrodex or Hodgdon 777 2F?